It is known (DE-A-1 542 544, DE-A-1 947 851, DE-A-2 106 644, DE-A-2 135 154, DE-A-2 214 056, DE-A-2 456 308, BE-PS-631 964, BE-PS-661 047, BE-PS-661 946, FR-PS 1 359 438 or GB-PS 768 111) that aromatic diamines can be prepared by catalytic hydrogenation of the corresponding aromatic dinitro compounds. The hydrogenation may take place under the combined use of solvents such as for example low-boiling alcohols such as methanol, ethanol or isopropanol, but may also be carried out without the use of such foreign solvents. The hydrogenation may be carried out with the aid of catalysts dispersed in the reaction mixture, which are then separated by sedimentation or filtration and optionally recycled to the process.
Up to now, the working-up of the reaction mixture was carried out in such a way that a mixture of aromatic diamines and water of reaction present after separation of the optionally co-used auxiliary solvent was first of all continuously freed from water under normal pressure in a distillation column and the diamine occurring as distillation residue was then optionally freed in further process steps from still adhering water and from organic impurities that were possibly still present. With this procedure mixtures of water with steam-volatile organic byproducts, such as occur in the hydrogenation of the dinitroaromatic compounds, are always formed as distillates. These byproducts are for example aromatic or cycloaliphatic monoamines and/or cycloaliphatic alcohols, i.e. for example in the case of the production of diaminotoluene, are toluidines, perhydrotoluidines and/or methylcyclohexanols.
These steam-volatile byproducts cause the overhead distilled water to be highly contaminated with these compounds. In EP 0236 839 B1, a process for the distillative working-up of such aqueous amine solutions is described, in which a waste water that is far less contaminated with organic impurities is obtained. To this end, the mixture is separated in a distillation column with side extraction. The vapors of the distillation column are condensed, and the liquid phase that is thereby formed is passed through a phase separation apparatus, in which steam-volatile organic byproducts are removed as organic phase from the vapor condensate. The aqueous phase is returned to the head of the distillation column. The water that is largely freed from steam-volatile organic impurities is removed via a side stream. The diamines freed from water and steam-volatile impurities are formed in this case as bottom product. A common feature of all these processes is, however, high energy consumption: 1.2 to 2 kg of heating steam have to be used per kg of water to be separated.